I'm not very good at explaining things, but, Flux figured out exactly what I am trying to accomplish.
The water, if left at a fixed nozzle size, could very well stall the rotor, if I can't match the electrical load to the rpm's/torque of the turbine. If just pumping water, that would not matter so much. Your supply of pumped water for usage would just slow to a trickle.
I got the sliding Alternator idea from watching a video of a Japanese hubmotor, being slowed and raising RPM's by sliding the stator in and out of the permanent magnet.
Also, I believe Harris had done some experimenting of moving/reducing the effects of magnetic flux in his Pelton systems.
I bought the blank covers, and the junction boxes do come in 3" and 4" round types. I will center drill them and weld them to the shaft. Then, I will rig a way to spin the assembly, and use my angle grinder to slowly grind them down to final dia. Then, I will grind rectanular openings for the magnets, and will need to cut a slot in each opening edge, to press the magnets into. This will prevent them from flying off the rotor.
The magnets are polarized through the thickness, so, I can just alternate each magnet to get the NSNSNS 6 pole rotor. The alternator has 6 N pole claw fingers and 6 S pole claw fingers. That's why I bought 7 magnets, allowing for screwing up one of them, somehow.
In my build thread, I explained how I had maximum water in a pair of 4" pies, running to the turbine. This will be max possibly 100 GPM potential of my system, and a 20GPM will be the minimum of my system. I have 26' of head once I get the weir constructed.
The original rotor I will leave original, so, I can play with each set up and get readings of outputs and RPM's, so I can try to figure out how to make the governing mechanism to move the Alternator housing.
The stator in the Alternator is .75" thick, so, I bought .75" long Neodymium N45 magnets. There will not be a lot of movement of the stator in this design, so, it will be a fine tuning type design, hopefully.
The Alternator is listed as a 25A rating, as far as I could find searching. It's a Denso from a Toyota, that I bought rebuilt from an Alternator repair shop near where I used to live. We kept burning out the external regulator, so, we switched to a 60A 1 wire Delco Alternator on our sawmill. That worked well, and, I had the Alternator laying on a shelf.
Thanks to both responders, especially Flux, for explaining what I thought I had figured out. I was not sure the idea had merit.
I will add a link to this thread in my build thread, unless an admin wants to move this thread into my build thread.